1. Field of the Invention
The present invention relates to a demodulation method and apparatus for performing a quasi-coherent detection of a modulated digital signal using a signal having a fixed frequency and compensating for the phase and frequency of a tentative demodulated signal obtained by the detection.
2. Description of the Related Art
It is well known that a modulated digital signal can be demodulated by, for example, a coherent detection system which is widely used for obtaining ideal demodulation characteristics. By the coherent detection system, a modulated digital signal s(t) is multiplied by cos .omega.ct (.omega.c: radian frequency of the carrier), a signal representing the result of the multiplication is filtered by a low-pass filter, and an output from the low-pass filter is used as a baseband signal.
In order to perform the coherent detection system, which requires a carrier, various technologies have been proposed, for example, of extracting a suppressed carrier included in a modulated wave from a modulated signal and reproducing the suppressed carrier. In general, a signal having a radian frequency .omega.c is generated using a demodulated code and the frequency is controlled by a VCO (voltage-controlled oscillator).
By a quasi-coherent detection system, to which the present invention is related, a modulated signal is detected using an oscillation signal having a fixed oscillation frequency which is substantially equal to the radian frequency .omega.c of the carrier and the phase of the tentative demodulated signal generated by the detection is controlled to obtain a determined demodulated signal.
One example of the quasi-coherent detection system is described in the U.S. Pat. No. 5,287,067.
The system described in the above-mentioned document operates in the following manner.
A complex demodulated signal which is received is treated by quadrature detection to generate a tentative complex demodulated signal. The correlation value between the complex demodulated signal and the complex identification signal is obtained as a final signal by the quasi-coherent detection system. Based on the correlation value, a frequency error of one symbol cycle (the cycle by which the tentative demodulated signal is input in repetition) is estimated. An initial phase error is estimated based on the frequency error, the tentative complex demodulated signal, and the demodulated determined complex signal. An optimum phase compensation amount is obtained based on the frequency error and the initial phase error. By compensating for the tentative complex demodulated signal in accordance with the optimum phase compensation amount, a determined complex demodulated signal is generated. Based on the determined complex demodulated signal, a complex identification signal is generated.
The above-described conventional system involves many points to be improved, such as, for example, reduction in pull-in time (time required to compensate for a phase shift or frequency error) when there is no training signal, improvement in signal quality, and simplification of the entire circuit system. For example, when there is no training signal, a circuit used for performing this system has a complicated configuration including a large number of complex multipliers. Moreover, the number of signals to be processed and the number of processes to be performed during one symbol cycle are excessive. In the case where such a circuit is used for high-speed digital transmission having a high transmission rate, the power consumption of the circuit is excessive.
According to the above-described system, the phase shift caused by the frequency error is estimated based on the tentative demodulated signal and the identification signal. Thus, when the identification signal performs erroneous identification due to a noise or the like, the pull-in time of the phase shift is prolonged and signal quality is deteriorated.
Furthermore, demodulation of a modulated signal accompanying amplitude change such as QAM (quadrature amplitude modulation) requires a circuit for normalizing the complex signal, an orthogonal coordinate--polar coordinate conversion circuit and a polar coordinate--orthogonal coordinate conversion circuit, which prolongs the signal processing time and enlarges the size of the circuit.